A cam on a camshaft has an outer surface which is normally formed by a part-cylindrical base region that is coaxial with the camshaft and with a lobe region that is eccentric to this axis. These regions must be machined to very tight standards, with the last operation being a honing of the surfaces to a near mirror finish, as in use in a motor vehicle the camshaft rotates at high speed while cam followers ride on the cam surfaces.
The honing operation as described in Lueger Lexikon der Technik (Deutsche Verlags, Stuttgart, volume 8 at pages 442 and 443) is carried out by urging fine-grit honing stones radially against the cams as the shaft is rotated about its axis. The stones have surfaces engaging the respective cams and each formed as a family of parallel lines parallel to the camshaft axis. These stones may also be reciprocated axially somewhat during the honing operation.
As described in German patents 3,011,454, 3,011,455, and 3,841,976 two stones are mounted in respective holders on the ends of respective arms at each cam, and these stones are diametrically opposed to each other to prevent the camshaft from being bowed. At the start of the grinding operation each stone surfaces is convex radially toward the camshaft so that it engages the respective cam surface along a contact line extending parallel to the camshaft axis. As the cam is rotated this contact line migrates somewhat on the stone surface as the stone surface engages the lobe region of the cam, but while the stone surface is engaged against the cylindrical base region it does not move at all. Thus in short order the stone wears away at the line where it spends most of its time in contact with the base region and takes on a shape concave toward the camshaft with a part-cylindrical recess on its face of the same diameter as the base region of the cam.
As a result the initially convex face of the grinding stone, which engages the workpiece in line contact and therefore has good material-removal characteristics, becomes a concave face that engages the workpiece in surface contact, with greatly reduced grinding efficiency.